Abstract
Low cycle fatigue properties under pure fatigue and creep/fatigue conditions were evaluated using a powder metallurgy superalloy, AF 115, containing a variety of defect-types and defect-sizes. The relationship between life, defect size, and loading condition were analyzed on the basis of crack growth rules and strain energy parameters. Following results were obtained ; (1) Inclusions, micro porosities and prior particle boundaries were observed at the origin of fatigue specimens. (2) By treating these defects as either cracks or notches, the lower or upper boundary of fatigue life can be obtained from the elastic-plastic strain energy parameter, lWf, and the fatigue crack growth rule. (3) The introduction of a tension hold period to the stress cycle or a decrease in the strain rate in the strain cycle significantly affects the fatigue life. The linear summation rule for partitioned lives, based on the elastic-plastic and creep strain energy parameters, lWf and lWc, and fatigue and creep crack growth rules, is suitable for life estimation under creep/fatigue conditions.
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